1. Field of the Invention
This invention relates generally to the field of petroleum reservoir management or other fields where movement of pore fluids through fractured rock is important, and more particularly to seismic fracture characterization methods. Specifically, the invention is a method for developing a rock physics model for layered fractured rocks to use in simulating seismic response.
2. Background of the Invention
Fractures may serve as major conduits for movement of pore fluids (hydrocarbons or water) and dissolved chemicals through low porosity, low permeability reservoir or non-reservoir rocks. Understanding fluid flow and mass transport in fractured rocks is essential for optimal reservoir management as well as other applications such as assessing the ground-water resources of hard-rock aquifers, investigating the suitability of underground sites for hazardous waste disposal, and predicting the movement of hazardous chemicals if contamination occurs. A key strategy for fractured reservoir management is a quantitative description of the geology, geophysics and petrophysical attributes.
Numerous laboratory and field studies indicate that aligned fractures will cause the anisotropic behavior of rock properties (such as wave velocities, attenuation, resistivity, thermal conductivity and permeability). Different fracture configurations/alignments will have different types of anisotropy. For example, a resulting effective medium of an isotropic rock matrix permeated with a single set of aligned fractures will be transversely isotropic (TI). Similarly, two or three orthogonally-intersected fracture sets will give an orthorhombic system. In more general case where two or more fracture sets intercept at arbitrary angles, one has a monoclinic system.
Quantifying fracture anisotropy with surface seismic data should provide an optimal strategy for fractured reservoir management by integrating the geophysical data from all scales with the engineering data. P-wave AVO/AVA and azimuthal velocity anisotropy analyses can be important for inferring fracture properties. In addition to these analyses, S-wave splitting (birefringence) of either pure S waves or converted PS waves can be analyzed to characterize special fracture distributions.
Rock physics modeling plays a key role in these seismic fracture characterization methods. Existing theories and workflows for modeling anisotropic elastic properties of a fractured rock are often of limited usefulness. For example, many widely-used fracture models (e.g. Hudson 1981) are valid only when fracture porosity is much smaller than the mean fracture aspect ratio. This is called the dilute pore concentration assumption and models with this limitation are hardly useful in real applications. Effective medium theories, such as differential effective medium theory (“DEM”) or self-consistent (“SC”) theory successfully overcome this limitation (e.g. Hornby et al. 1994).
However, inclusion-based effectively medium theories (e.g. DEM and SC) have their own problems. First, one key assumption in these models is that all pores or fractures are isolated, which does not allow the fluid communication between fractures and matrix pores. This will give a high-frequency response in terms of fluid flow mechanisms. Therefore these models are not suitable to simulate low-frequency seismic responses. Theoretical studies (e.g. Thomsen 1995, Xu 1998) and laboratory experiments (e.g. Rathore et al. 1995) show that fluid communication can have a significant effect on the resulting seismic anisotropy. It is very important to correctly handle fluid communication to simulate seismic anisotropy. One solution to the problem is to first calculate dry rock frame properties using the inclusion-based models and then to introduce the pore fluid into the system using Gassmann's theory (e.g. Xu and White, 1995). This gives the low-frequency response since Gassmann's theory assumes a totally connected pore system.
Another drawback with the DEM or SC theory is that they are computationally extremely expensive. In some cases, it becomes impractical to apply their anisotropic versions, for example, to well log analysis. Keys and Xu (2002) proposed a dry rock approximation method, which dramatically speeds up the numerical calculation of the differential effective medium method while maintaining its accuracy. Xu, Saltzer and Keys (PCT Patent Application Publication WO/2006/062612) extended the dry rock approximation to the anisotropic case. Above all, all the fracture models mentioned above are designed to estimate elastic properties of fractured rocks only. They are unable to handle rock columns with mixed anisotropy types (e.g. shale anisotropy and layering anisotropy mixed with fracture anisotropy). In real cases, a fractured rock layer often has limited thickness and coexists with other rock types, such as shale (FIG. 1). At a typical seismic frequency, say 30 Hz, the wavelength can be as large as 100 meters. If the overall thickness of the mixed rock column is less than, for example, half of the wavelength, it is not possible to separate the fracture anisotropy from shale anisotropy and layering anisotropy. In practical applications, therefore, it is critical to have a rock physics model that can consistently consider various types of geological factors including, but not limited to, porosity, shale volume, fluid content, lithology, pore type and fractures when modeling seismic response.
Furthermore, to improve the seismic modeling efficiency, a plurality of logs sampled at a constant sampling interval such as 0.5 foot are often blocked into a limited number of layers in a consistent manner using a theoretical model. This is called seismic upscaling. Backus averaging (1962) is able to handle individual layers with anisotropy up to transversely isotropic with a vertical symmetry axis (“VTI”). The resulting effective medium always exhibits a VTI symmetry. The Backus method has been widely used in a sand-shale system. However, the method is unable to handle a geological system with types of anisotropy other than VTI. This is because in many geological conditions, fractures typically align vertically and, hence, give a HTI (transverse isotropy with a horizontal symmetry axis) for a single fracture set, or orthorhombic for two orthogonally-intersected fractures. In more general cases where two or more fracture sets intersect obliquely or a VTI medium (e.g. shale) having one set of non-vertical fractures, one has a monoclinic system. In short, a more sophisticated scheme is needed for upscaling elastic properties of a geological rock column with mixed fractured rocks and other geological bodies. The present invention satisfies this need.